The present invention is directed to a press section for a fibrous web, such as a paper web, and to a method of applying pressure therein onto the fibrous web passing through such a press section.
More specifically, the present invention is directed to a press section having a close draw, such as for a paper machine or a cardboard machine, in which at least two press zones are formed, at least one of the press zones being a belt nip and/or a shoe nip, having a wide pressing region in the direction of the running of the fibrous web.
It is commonly known in the prior art to remove water out of fibrous webs, such as from paper or cardboard webs, by passing such a web through a press nip formed by two rolls disposed opposite one another. As known in the prior art, one or two press fabrics and passed through such dewatering nips, these press fabrics carrying water removed out of the fibrous web, and also acting to convey the fibrous web forwardly.
As the production rate of such paper machines increases, the dewatering performed by means of such nip pressing has become an obstacle limiting increase of speed of paper production. This is due to the fact that the press nip is formed by a pair of rolls having a short pressing area, so that the residence time of a running fibrous web in these nips is very short at very high speeds of operation. However, due to the flow resistance of the fibrous structure of the web itself, water requires a certain minimum amount of time for escaping out of the web into the hollow face of a press roll or into a press fabric, in a dewatering operation.
Several successive press nips have been used for dewatering as disclosed in the prior art, examples of which include the so-called compact "Sym-Press" press section, or several separate, individual, successive press nips. However, the use of successive press nips requires a relatively large area for operation, especially if separate, individual press nips are used one right after the other. Compact construction of press sections however, such as the "Sym-Press" press section, cause difficulty in obtaining optimal positioning of the various components, as well as creating difficulties in the operation of the press itself, such as in the removal of paper broke. In nip presses, suction rolls are commonly used which are relatively expensive components and which consume a tremendous amount of suction energy and cause noise. In suction rolls, a perforated mantle must be used, which causes problems with the mechanical strength of such suction rolls.
If an attempt is made to increase dewatering output in nip presses by increasing the nip pressure, a certain limiting line pressure value is reached, beyond which any increase in the line pressure is no longer helpful because the structure of the fibrous web and of the press fabrics can no longer withstand the increased compression pressure.
Attempts have been made to lengthen the area of roll nips contacted by the web to be dewatered, through the use of rolls of larger diameter and by using soft press fabric, but even with utilization of these features, a limit in terms of feasible economic application is soon reached.
In order to solve the problems noted above in addition to other problems, so-called long-nip presses have been developed in recent years. Embodiments of such presses are disclosed, for example, in U.S. Pat. Nos. 3,808,092; 3,808,096; 3,840,429; 3,970,515; 4,201,624; and 4,229,253, as well as in GP Pat. Appln. No. 2,057,027.
Another prior art press construction is disclosed in U.S. Pat. No. 3,783,097, assigned to Beloit Corporation, U.S.A.
In U.S. Pat. No. 3,783,097, a long-nip press is disclosed in which several subsequent pressure shoes are utilized, the pressure shoes being pressed towards an opposing belt and press roll.
In U.S. Pat. No. 3,840,429, a long-nip press is described in which the web to be pressed linearly runs between two felts through a press zone formed by two opposing press shoes, and generated by means of a pressure medium. Moreover, bands are disposed inside the loops of the felts, to define the press zones and transmit pressure of the medium to the web.
In the prior art roll presses, it has generally been necessary to use a press suction roll, which results in considerable drawbacks, to be discussed in more detal infra. The perforations of a press suction roll may leave markings on the web which impairs the appearance of the paper, and which may also affect the surface properties of the paper produced. Press suction rolls are costly, require a drive motor of their own with concomitant adjustment systems, and also cause a tremendous amount of noise. In particular, drilling and mounting of such press suction rolls is a difficult operation, and also results in high costs. The perforations of on the press suction roll reduces the strength of the mantle. It therefore is necessary to use special metal alloys as the construction material for the rolls, as well as constructing such rolls with mantles of large thicknesses, also resulting in high material costs.
Press suction rolls consume large amounts of air, because in addition to air passing through the web and the felt entering the suction system, ambient air also enters into the suction system through the holes in the mantle along the suction zone of the suction roll, during each revolution of the roll. Moreover, sealing water of the suction box in the suction roll causes many difficulties.
An operational drawback of the suction rolls is that they operate at high noise levels. Reduction of such noise in a press section is also an object of the present invention.
In particular, in the case of press suction rolls, it is often necessary to utilize variable crown equipment. However, this is usually not possible because the mantle of the suction roll is perforated and/or the interior of the suction roll is occupied by the suction box to such an extent, that conventional variable crown equipment cannot be fitted therein. As noted above, a suction roll, especially a press-suction roll, is an extremely expensive component, and may cost up to about 3,000,000 FIM, at present day values.